1. Field of the Invention
The present invention relates to a highly heat-resistant and hydrothermal-resistant .beta.-zeolite and an adsorbent for automobile exhaust gas purification using the .beta.-zeolite.
2. Description of the Related Art
In order for a catalyst (used for purification of the exhaust gas emitted from an automobile or the like) to exhibit its catalytic activity, the catalyst must be heated to an activation temperature or higher by, for example, the heat of the exhaust gas. When the temperature of the exhaust gas is low as in the cold start of an engine, the harmful substances in the exhaust gas, such as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) are hardly purified. The purification of, in particular, HC which is discharged in a large amount during the cold start of an engine, is an important task to be achieved.
In order to improve the efficiency of HC purification during the cold start, there are known techniques of using, as an HC adsorbent, a molecular sieve made of a crystalline aluminosilicate such as zeolite or the like and allowing the adsorbent to adsorb HC while a catalyst used together reaches its activation temperature.
For example, in Japanese Patent Application Kokai (Laid-Open) No. 75327/1990 is disclosed an apparatus for automobile exhaust gas purification, using a Y type zeolite or mordenite as an HC adsorbent. Also in Japanese Patent Application Kokai (Laid-Open) No. 293519/1992 is disclosed use of an HC adsorbent obtained by subjecting a H.sup.+ /ZSM-5 zeolite to ion exchange with Cu and Pd, in order to alleviate the adverse effect of water adsorption, attain improved HC adsorption capability, and widen the temperature range for HC adsorption. For the same purpose, use, as an HC adsorbent, of a pentasil type metallosilicate subjected to ion exchange with H, Cu or Pd is proposed in Japanese Patent Application Kokai (Laid-Open) No. 63392/1994.
Further in Japanese Patent Application Kokai (Laid-Open) No. 99217/1997 is proposed use, as an HC adsorbent, of an H.sup.+ /.beta.-zeolite having a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of 100 or more, superior in HC adsorption capability and capable of maintaining its pore structure even when subjected to an exhaust gas of 750.degree. C. or higher.
The SiO.sub.2 /Al.sub.2 O.sub.3 ratio indicative of the composition of zeolite framework and/or the ion (e.g. H.sup.+, Na.sup.+ or Cu.sup.2+) presence close to the Al of zeolite framework for electric charge compensation, which as the yardstick for the heat resistance of zeolite, have heretofore been used mainly.
It was discovered, however, that two zeolites having the same SiO.sub.2 /Al.sub.2 O.sub.3 ratio or the same ion have greatly different heat resistances when they are produced from different raw materials or different processes.
When a zeolite insufficient in heat resistance, particularly hydrothermal resistance (e.g. heat resistance in automobile exhaust gas) is used for purification of the exhaust gas emitted from an internal combustion engine of an automobile or the like, the pore structure of the zeolite is gradually collapsed; therefore, such it is found that a zeolite will deteriorate in purifiability when used in a high temperature exhaust gas such as emitted during continuous high-speed engine operation or the like.
Also, when such a zeolite is used in a catalyst and subjected to a heating and regeneration treatment for removal of formed coke or the like, there have been cases that the pore structure of the zeolite is impaired.
Thus, it has been a task how to define a highly heat-resistant and hydrothermal-resistant zeolite by its properties and produce such a zeolite.